A method is provided for improving high temperature performance grade properties of polymer-modified, vulcanized asphalt by treating an asphalt cement under mild oxidizing conditions sufficient to provide a change in viscosity per hour as measured at 140.degree. C. of less than about 500 poises per hour, of said asphalt cement, adding 1 wt. % to 10 wt. % of a polymer based on weight of said asphalt cement, and vulcanizing the polymer-containing asphalt cement. The invention further relates to asphalt compositions thus made and paving compositions containing these asphalt compositions.